1. Field of the Invention
The present invention relates to a switching power supply, reducing a loss when a rectifying element is conducting, to improve a power conversion efficiency.
2. Technical Background
Although a diode has been entirely used as a rectifying element for converting an A.C. voltage or current into D.C., in case of being rectified through the diode, it must be taken into consideration that a loss occurs, corresponding to the product of a forward dropping voltage V.sub.F and current in the diode.
On the other hand, from an object to reduce the loss of the rectifying element, as a saturated voltage between a collector and an emitter of a transistor being in an ON-state is less than the forward dropping voltage V.sub.F of the diode, it is known that the resulting loss becomes low.
Then, it has been considered that a transistor instead of a diode for the rectifying element may be used as a means for the improving power conversion efficiency of the switching power supply.
With respect to a switching power supply in which such a transistor rectifying element is used, inventors of the present application have proposed a switching power supply as shown in FIG. 1 of the present application in Japanese Patent Application No. 4-105807.
Detailed description of structures and operation of the circuit as shown in FIG. 1 is omitted, but the operation of a transistor Q2 as the transistor rectifying element is as follows:
When a switching transistor Q1 turns off, a current flowing through a path formed by an input terminal 1, a choke coil L1, a collector and an emitter of the switching transistor Q1 is cut off. Thus, a flyback voltage is generated in the choke coil L1, and a collector voltage of the switching transistor Q1 rises up to a voltage that the flyback voltage is added to an input voltage V.sub.in. When a voltage that the flyback voltage and the input voltage V.sub.in are superposed inputs to an emitter of the transistor Q2, the transistor Q2 is forwardly biased and becomes an ON-state. When the transistor Q2 becomes on-state, the high voltage that the flyback voltage and the input voltage V.sub.in is superposed is supplied to a load RL through an output terminal 2.
On the contrary, when the switching transistor Q1 turns on, the collector voltage thereof and the emitter voltage of the transistor Q2 become low, and a forward bias between an emitter and a collector of the transistor Q2 is released and the transistor Q2 becomes an OFF-state. Like this, the transistor Q2 becomes the ON-state by that the switching transistor Q1 turns off and its output, that is, the collector voltage rises, and conversely, the transistor Q2 becomes the OFF-state by that the switching transistor Q1 turns on and its output, that is, the collector voltage drops. After all, in the circuit as shown in FIG. 1, the output of the switching transistor Q1 has become a signal for turning the transistor Q2 on and off.